1. Field of the Invention
The present invention relates to a light deflector, and more particularly to a waveguide-type light deflector for deflecting a light beam that has been guided through an optical waveguide through diffraction caused by a surface elastic wave and for issuing the deflected light beam out of the optical waveguide.
The present invention is also concerned with a waveguide-type light deflector for diffracting a light beam that has been guided through an optical waveguide, with a surface elastic wave propagated in the optical waveguide, and for controlling the energy intensity of the surface elastic wave to vary the diffraction efficiency, thereby to modulate the light beam.
2. Description of the Prior Art
Light scanning recording apparatuses or light scanning reading apparatuses, for example, employ light deflectors for deflecting a light beam. Such light deflectors include mechanical light deflectors, such as galvanometer mirrors or polygon mirrors, electrooptic deflectors (EODs), and acoustooptic deflectors (AODs). Mechanical light deflectors are problematic in that they have poor durability and a large size. EODs and AODs are disadvantageous in that they have long light beam paths since the angle at which they deflect light cannot be large, and hence light scanning recording or reading apparatuses incorporating them are large in size.
One light deflector which has recently been proposed to solve the above drawbacks employs a light waveguide. The proposed light deflector has a slab-shaped light waveguide made of a material capable of propagating a surface elastic wave and means in the optical waveguide for generating a surface elastic wave which travels across a light beam guided in the optical waveguide, the surface elastic wave having a continuously variable frequency. The means for generating a surface elastic wave comprises, for example, an interdigital transducer (IDT), and a driver for applying an alternating voltage with a continuously variable frequency to the interdigital transducer. In this light deflector, since the light beam guided in the optical waveguide undergoes Bragg diffraction due to an acoustooptic interaction between the light beam and the surface elastic wave, and also since the angle of diffraction is variable and depends on the frequency of the surface elastic wave, the light beam can be continuously deflected in the optical waveguide by varying the frequency of the surface elastic wave. A light deflector of this type is described in detail in U.S. Pat. No. 4,778,991, for example.
As described above, a waveguide-type light deflector generally employs an IDT as a means for generating a surface elastic wave. The IDT is disposed to the side of the path over which light beams guided through the optical waveguide travel so that the IDT will not directly interfere with the light beams.
A waveguide-type light deflector of the above design is required to increase the width of a light beam guided through the optical waveguide as much as possible. More specifically, where a deflected light beam issued from the optical waveguide is used to scan a recording medium, for example, to record a desired image thereon, the width of the light beam must be increased while being guided through the optical waveguide and then be converged into a light beam spot in order to increase the resolution of the recorded image.
If the light beam guided through the optical waveguide has an increased width, however, the surface elastic wave generated from the IDT, located as described above, has to traverse an increased distance in order to cross the light beam, during which time the surface elastic wave is greatly attenuated by being absorbed by the optical waveguide. The energy intensity I of the surface elastic wave as it is propagated in the optical waveguide is expressed by the following equation: EQU I=Io e.sup.-.alpha.x... (1)
where Io is the energy intensity of the surface elastic wave when it is generated by the IDT, o is the coefficient of ultrasonic absorption by the optical waveguide, and x is the distance from the IDT. Equation (1) indicates that the surface elastic wave intensity I is exponentially attenuated as the distance x increases [see FIG. 3(a)]. The efficiency .eta. of diffraction of the light beam by the surface elastic wave is given as follows: EQU .eta.=sin.sup.2 A.sqroot.I
where A is a coefficient. The diffraction efficiency .eta. is lowered as the surface elastic wave intensity I is lowered. Accordingly, the diffraction efficiency .eta. is greatly reduced as the distance x from the IDT increases. If the intensity of the light beam before it is diffracted by the surface elastic wave has a Gaussian distribution as indicated by the solid-line curve in FIG. 3(b), then the diffracted light beam has an intensity distribution as indicated by the dotted-line curve in FIG. 3(b). More specifically, since the surface elastic wave is largely attenuated before it reaches a central position across the width of the light beam, where the light beam intensity P is maximum at Po, the intensity across the entire width of the diffracted light beam is considerably reduced. Further, the intensity distribution of the diffracted light beam becomes quite different from the Gaussian distribution which the light beam exhibited before being diffracted, so that difficulty will be experienced in sufficiently converging the diffracted light beam.
There is also known a waveguide-type light modulator, which similarly to the above waveguide-type light deflector, has a slab-shaped light waveguide and means in the optical waveguide for generating a surface elastic wave which travels across a light beam guided in the optical waveguide. In this light modulator, since the light beam guided in the optical waveguide undergoes Bragg diffraction due to an acoustooptic interaction between the light beam and the diffraction is variable and depends on the energy intensity of the surface elastic wave, the light beam can be modulated by controlling the energy intensity of the surface elastic wave.
The waveguide-type light modulator also employs an IDT as a means for generating a surface elastic wave. The surface elastic wave intensity can be controlled by continuously controlling the level of an alternating voltage applied to the IDT or by turning on and off such an alternating voltage applied to the IDT. The IDT is disposed to the side of the path over which light beams guided through the optical waveguide travel so that the IDT will not directly interfere with the light beams.
The waveguide-type light modulator of the foregoing arrangement is also required to increase the width of the light beam guided through the optical waveguide as much as possible in order to increase the resolution of a recorded image. Efforts to meet this requirement, however, result in the same problems as those described above with respect to the waveguide-type light deflector. More specifically, the intensity of the diffracted light beam is reduced to a large extent, and the intensity distribution of the diffracted light beam is widely different from a Gaussian distribution, making it difficult to sufficiently converge the light beam.
If the light beam guided through the optical waveguide has an increased width, then the period of time required for the surface elastic wave to travel across the guided light beam is increased, and hence it becomes impossible to achieve a sufficient modulation rate.